1. Field of the Invention
The present invention relates to a refrigeration system and, more particularly, to a multistation cryogenic refrigeration system.
2. Description of the Prior Art
The prior art and the present invention will be described for use in connection with masers. However, it will become apparent from the following description that the invention is not intended to be limited thereto. The need to provide refrigeration at cryogenic temperatures for the proper operation of masers, such as the microwave masers at S, X and K.sub.u band frequencies, is well known. 4.5.degree. K closed-cycle refrigeration systems (CCRs) are currently used in the Deep Space Network for such masers. Such refrigeration systems have been described in the prior art. Attention is directed to but two prior art references.
(A) "A Simplified Approach to Heat Exchanger Construction for Cryogenic Refrigerators," by W. H. Higa and E. Wiebe, Cryogenic Technology, Vol. 3, pp 47051, March/April 1967.
(B) "Low Noise Receivers: Microwave Maser Development" by R. Clauss, E. Weibe and R. Quinn, in The Deep Space Network Progress Report, Technical Report 32-1526, Vol. XI, pp. 71-80, Jet Propulsion Laboratory, Pasadena, Calif., Oct. 15, 1972.
As described in reference (b), an X-band maser achieved 45dB net gain and had a theoretical noise temperature of 3.5.degree. K (defined at the maser input connection at the final stage of refrigeration), when operated at 4.5.degree. K. A CCR, providing refrigeration at about 4.5.degree. K is described in reference (a) as well as in U.S. Pat. No. 3,421,331 issued with respect to an invention of Walter H. Higa on Jan. 14, 1969.
To date in known CCRs, used with masers, the coldest station is at about 4.5.degree. K which is the temperature at which most of the maser's parts, such as the large superconductive magnet, has to be maintained for satisfactory performance. It is appreciated that if the temperature of the maser's amplifier, such as the ruby in a ruby maser, were reduced from 4.5.degree. K to 3.0.degree. K, the maser noise would decrease from 3.5.degree. K to 2.0.degree. K, resulting in a gain increase from 45dB to 72dB. Such gain increase can be traded for additional bandwidth.
It is recognized that the basic known CCR may be operated to provide a coldest station of 3.0.degree. K, rather than at 4.5.degree. K. However, to provide a coldest station at 3.0.degree. K to cool all the maser's parts, including those of high heat capacity, a very large vacuum pump would be required, which would greatly increase the cost and size of the CCR. Since except for the maser amplifier all other parts require a lowest temperature of only 4.5.degree. K, rather than at 3.0.degree. K, in order to reduce the CCR's size, complexity and cost, even though higher gain could have been attained if the CCR were to provide refrigeration at 3.0.degree. K. A need therefore exists for a CCR in which 3.0.degree. K refrigeration is provided without significantly increasing the size, complexity and cost of the CCR.